The application concerns a reflection-reducing layer on highly reflecting surfaces and a process for the preparation of such layers.
It is known to prepare masks, diaphragms, slits, gratings or coding arrangements by applying them in the form of a thin, impermeable layer to a transparent carrier, for example, lenses or plane parallel plates. This is effected in most cases by vapor deposition in a high vacuum or by sputtering. Metals, particularly aluminum or chromium, have been found suitable as coating materials.
A disadvantage of the method is the relatively high reflectivity at the interfaces between the metal and the adjacent media, because the reflections produced may lead to various interferences, such as for example contrast reductions, light scattering or signal falsification.
In the case of a layer with an optical thickness of .lambda..sub.o /4 the following condition must be satisfied to completely eliminate reflection components. ##EQU1## wherein:
n.sub.1 --the refractive index of an absorption-free incident medium,
n.sub.2 --the refractive index of an absorption-free reflection-reducing layer,
n.sub.3 --the real portion of the refractive index of the highly reflective material,
k.sub.3 --the absorption coefficient of the highly reflective material and
.lambda..sub.o --the wave length at which the reflection disappears.
The refractive indices resulting from this equation for the coating materials are, however, substantially higher than those available for practical applications.
In order to overcome this objective difficulty, it has already been proposed to provide multiple layers in place of a single coating, which consist, for example, of a plurality of .lambda..sub.o /4- and .lambda..sub.o /2- layers and the refractive indices of which are adjusted to the optical constants of the highly reflective material and the adjacent medium. The disadvantage of this proposed solution consists of the fact that the effective spectral range is relatively narrow.